1. Field of the Invention
The present invention relates to a superconducting wire and manufacturing method thereof, as well as to an oxide superconducting coil and a cable conductor. More specifically, the present invention relates to an oxide superconducting wire, manufacturing method thereof, an oxide superconducting coil and a cable conductor having high critical current density and low a.c. loss.
2. Description of the Background Art
In recent years, superconducting materials of ceramics, i.e., oxide superconducting materials, are watched with interest due to higher critical temperatures thereof. Among these materials, yttrium, bismuth and thallium oxide superconducting materials which exhibit high critical temperatures of about 90K, 110K and 120K respectively, are expected for practical application.
A single-filamentary oxide superconducting wire having high critical current density formed of such oxide superconducting materials is obtained by heat treating and then covering with a metal sheath the material powder, drawing, rolling and by further heat treatment. Similarly, an oxide superconducting multi-filamentary wire having high critical current density is obtained by heat treating powder mainly consisting of oxide superconducting material, then covering the same with a metal sheath, drawing and putting together the as-obtained wires to provide a multi-filamentary wire, and further by drawing, rolling and heat treating the same.
It has been conventionally known that an oxide superconducting wire having higher critical current density can be obtained by repeating several times the steps of rolling and heat treatment in manufacturing such an oxide superconducting wire.
If such an oxide superconducting wire is to be applied to an a.c. cable or magnet, it must have low a.c. loss, high strength and superior property under bending-strain, in addition to the high critical current.
The single-filamentary and the multi-filamentary oxide superconducting wires manufactured through the conventional method described above have as high a critical density as 30000 A/cm.sup.2 or higher.
However when an a.c. current is applied with the wire wound in a coil, there is generated an a.c. loss heat radiation. This is because an induced current flows between the metal sheath and the ceramics when an a.c. current is applied, resulting in heat radiation caused by a.c. loss of normal conduction resistance of the metal sheath, as compared to the case when a d.c. current is applied, in which case current flows only through the ceramic portions. Since the temperature of the coil as a whole increases, critical current density is decreased.
Accordingly, the operational frequency of the coil manufactured in accordance with the conventional method has been about 0.1 Hz at the highest.